ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



227 



C3) Illuminating- and other Apparatus. 



Zehnder's New Half-shadow Polarimeter.* — L. Zehnder reports 

 that this half-shadow polarimeter has proved itself very useful for the 

 examination of elliptically polarised light. The chief parts are shown 

 in fig. 22. The goniometer used in its construction was von Lang's. 

 The parallelised rajs, proceeding from the objective of the slit-tube 

 C, pass through the polariser P. They then traverse the Soleil-Babinet 

 compensator K, and the new half-shadow analysing arrangement A. All 

 these parts are in front of the telescope objective. The polariser P is 

 rotatory about the slit-tube axis, and a finely divided circle 1\ is closely 

 connected with it. This circle is coarsely adjustable, and also, by 

 means of the screw S x , finely adjustable ; the rotation is read off in 

 degrees and minutes by a vernier. The compensator K is intended to 

 convert into a directly polarised beam the light reflected at the surface 

 under examination, and more or less elliptically polarised, of the body 

 set on the goniometer. The compensator consists (i.) of a plane-parallel 



Fig. 22. 



quartz plate c of uniform thickness, and (ii.) of two quartz wedges 

 a and b, which together form a plane-parallel plate of variable thickness. 

 The optical axes of a and b, on the one hand, and of c on the other, are 

 orientated in the well-known manner, perpendicular to each other. The 

 half-shadow analysing arrangement A consists of the half-prism N" and 

 of the plane-parallel dark glass R, which together half cover up the 

 held of view defined by the circular diaphragm D. The divided circle 

 T 2 is closely connected with this analysing arrangement, and can be 

 rotated about the telescope axis ; the movement is partly coarse and 

 partly, by means of the screw S 2 , fine ; the vernier reads to degrees and 

 minutes. The opacity of the dark glass plate is so selected that the 

 diaphragm D under the light used is perfectly visible, and for this 

 reason the ocular lens of the telescope F is made of variable width by 

 means of an ocular slit. The well-known conditions of the Lippich 

 half -shadow polarising apparatus are made use of in adjusting the half- 

 prism and the dark glass. As the dark glass in each rotation of the 

 analyser about the telescope axis transmits a uniform quantity of light, 

 there occurs on each side of every dark adjustment of the half -prism 

 an adjustment for half-shadow equality — that is, for uniform brightness 



* Ann. d. Physik., xxvi. (1908) p. 985. 

 1909) pp. 29G-8 (1 fig.). 



See also Zeit. f. Instrumentenk., xxix. 



